<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
</head>
<body>
<div id="app">
    <p>{{ foo }}</p>
    <button v-on:click="foo = 'baz-111'">Change it</button>
</div>
</body>
<script>
    var obj = {
        foo: 'bar'
    }

    Object.freeze(obj)

    new Vue({
        el: '#app',
        data: obj
    })
</script>
</html>